International patent application PCT/GB2006/002471 (WO2007/007041) discloses the Agent in 2 different crystalline forms (Example 39k). One was crystallised from acetonitrile and had a melting point (melting onset) 108.5° C. This form will hereinafter be referred to as Form A. The other crystalline from described on page 176 of WO2007/007041 had a melting point (melting onset) of 113.8° C. This form will hereinafter be referred to as Form A′. The preparation of Form A is also described in the Example hereinafter. Form A and Form A′ convert to the amorphous form in aqueous media. The amorphous form has a different solubility profile to the Form A. Stable crystalline forms that do not convert to other forms with different solubilities in aqueous media are preferred for pharmaceutical formulations.